Implantable substrate coated with a macromer having free radical polymerizable substituents

ABSTRACT

Water soluble macromers are modified by addition of free radical polymerizable groups, such as those containing a carbon-carbon double or triple bond, which can be polymerized under mild conditions to encapsulate tissues, cells, or biologically active materials. The polymeric materials are particularly useful as tissue adhesives, coatings for tissue lumens including blood vessels, coatings for cells such as islets of Langerhans, coatings, plugs, supports or substrates for contact with biological materials such as the body, and as drug delivery devices for biologically active molecules.

This is a continuation of application Ser. No. 09/694,836, filed Nov.23, 2000, now U.S. Pat. No. 6,632,446, which is a continuation ofapplication Ser. No. 09/033,871, filed Mar. 3, 1998, now U.S. Pat. No.6,465,001, which is a continuation of application Ser. No. 08/467,693,filed Jun. 6, 1995, now U.S. Pat. No. 5,834,274, which is a divisionalof application Ser. No. 08/024,657, filed Mar. 1, 1993, now U.S. Pat.No. 5,573,934, which is a continuation-in-part of application Ser. No.07/958,870, filed Oct. 7, 1992, now U.S. Pat. No. 5,529,914, which is acontinuation-in-part of application Ser. No. 07/870,540, filed Apr. 20,1992, now abandoned.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to methods for coating and/orencapsulating surfaces and three-dimensional objects with cross-linkednetworks of water-soluble polymers.

Microencapsulation technology holds promise in many areas of medicine.For example, some important applications are encapsulation of cells forthe treatment of diabetes (Lim, F., Sun, A.M. “Microencapsulated isletsas bioartificial endocrine pancreas”, (1980) Science 210, 908–910),encapsulation of hemoglobin for red blood cell substitutes, andcontrolled release of drugs. However, using the prior art methods, thematerials to be encapsulated are often exposed to processing conditions,including heat, organic solvents and non-physiological pHs, which cankill or functionally impair cells or denature proteins, resulting inloss of biological activity. Further, even if cells survive theprocessing conditions, the stringent requirements of biocompatibility,chemical stability, immunoprotection and resistance to celluarovergrowth, of the encapsulating materials restrict the applicability ofprior art methods.

For example, the encapsulating method based on ionic crosslinking ofalginate (a polyanion) with polylysine or polyornithine (polycation)(Goosen, et al., (1985) Biotechnology and Bioengineering, 27:146) offersrelatively mild encapsulating conditions, but the long-term mechanicaland chemical stability of such ionically crosslinked polymers remainsdoubtful. Moreover, these polymers when implanted in vivo aresusceptible to cellular overgrowth (McMahon, et al., (1990) J. Nat.Cancer Inst., 82(22), 1761–1765) which over time restricts thepermeability of the microcapsule to nutrients, metabolites and transportproteins from the surroundings. This has lead to starvation and death ofencapsulated islets of Langerhorns (O'Shea, G. M. et al. (1986)Diabetes, 35:943–946).

Thus, there remains a need for a relatively mild cell encapsulationmethod which offers control over properties of the encapsulating polymerand yields membranes in the presence of cells which are permselective,chemically stable, and very highly biocompatible. A similar need existsfor the encapsulation of biological materials other than cells andtissues, as well as materials contacting biological materials.

Materials are considered biocompatible if the material elicits either areduced specific humoral or cellular immune response or does not elicita nonspecific foreign body response that prevents the material fromperforming the intended function, and if the material is not toxic uponingestion or implantation. The material must also not elicit a specificreaction such as thrombosis if in contact with the blood.

Gels made of polymers which swell in water to form a hydrogel, such aspoly(hydroxyethyl methacrylate) (poly(HEMA)), water-insolublepolyacrylates, and agarose, have been shown to be useful forencapsulating islets and other animal tissue (Iwata, et al., (1989)Diabetes, 38:224–225; Lamberti, et al., (1984) Appl. Biochem. Biotech.,10, 101–105 (1984). However, these gels have undesirable mechanicalproperties. Agarose forms a weak gel, and the polyacrylates must beprecipitated from organic solvents, which are potentially cytotoxic.Dupuy, et al. (1988) have reported the microencapsulation of islets bypolymerization of acrylamide to form polyacrylamide gels. However, thepolymerization process requires the presence of toxic monomers such asacrylamide and cross-linkers, and, if allowed to proceed rapidly tocompletion, generates local heat.

Microcapsules formed by the coacervation of alginate and poly(L-lysine)have been shown to be immunoprotective, for example, as described byO'Shea, et al., 1986. However, a severe fibrous overgrowth of thesemicrocapsules was observed following implantation (McMahon, et al. 1990;O'Shea, et al., 1986). The use of poly(ethylene oxide) (PEO) to increasebiocompatibility is well documented in literature. The biocompatibilityof algin-poly(L-lysine) microcapsules has been reported to besignificantly enhanced by incorporating a graft copolymer of PLL and PEOon the microcapsule surface (Sawhney, et al., “Poly(ethyleneoxide)-Graft-Poly(L-Lysine) Copolymers to Enhance the Biocompatibilityof Poly(L-Lysine)-Alginate Microcapsule Membranes,” (1991) Biomaterials,13 863–870).

The PEO chain is highly water soluble and highly flexible. PEO chainshave an extremely high motility in water and are essentially non-ionicin structure. Immobilization of PEO on a surface has been largelycarried out by the synthesis of graft copolymers having PEO side chains(Sawhney, et al.; Miyama, et al., 1988; Nagoaka, et al.). This processinvolves the custom synthesis of monomers and polymers for eachapplication. The use of graft copolymers, however, still does notguarantee that the surface “seen” by a macromolecule consists entirelyof PEO.

Electron beam cross-linking has been used to synthesize PEO hydrogels,which have been reported to be non-thrombogenic by Sun, et al., (1987)Polymer Prepr., 28:292–294; Dennison, K. A., (1986) Ph.D. Thesis.Massachusetts Institute of Technology. However, use of an electron beamprecludes including with the polymer any living tissue since-theradiation is cytotoxic. Also, the networks produced by this method aredifficult to characterize due to the non-specific cross-linking inducedby the electron beam.

Photopolymerization of PEG diacrylates in the presence of shortwavelength ultraviolet light initiation has been used to entrap yeastcells for fermentation and chemical conversion (Kimura, et al. (1981),“Some properties of immobilized glycolysis system of yeast infermentative phosphorylation of nucleotides,” Eur. J. Appl. Microbio.Biotechnol., 11:78–80; Omata et al., (1981), “Steroselectic hydrolysisof dl-methyl succinate by gel-entrapped Rhodotorula minuta uzr. texensiscells in organic solvent,” Eur. J. Appl. Microbial Biotechnol,11:199–204; Okada, T., et al., “Application of Entrapped Growing YeastCells to Peptide Secretion System,” Appl. Microbiol. Biotechnol., Vol.26, pp. 112–116 (1987). Other methods for encapsulation of cells withinmaterials photopolymerizable with short wavelength ultraviolet radiationhave been used with microbial cells (Kimura, et al., 1981; Omata, etal., 981; Okada, et al., 1987; Tanaka, et al., 1977; Omata, et al.,1979a; Omata, et al., 1979b; Chun, et al., 1981; Fukui, et al., 1976;Fukui, et al., 1984). However, yeast cells and some microbial cells aremuch hardier and resistant to adverse environments, elevatedtemperatures, and short wavelength ultraviolet radiation than mammaliancells and human tissues.

There are several problems with these methods, including the use ofmethods and/or materials which are thrombogenic or unstable in vivo, orrequire polymerization conditions which tend to destroy living mammaliantissue or biologically active molecules, for example, short wavelengthultraviolet radiation. In order to encapsulate living tissue forimplantation in a human or other mammalian subject, the polymerizationconditions must not destroy the living tissue, and the resultingpolymer-coated cells must be biocompatible.

There is also a need to encapsulate materials within a very thin layerof material that is permeable to nutrients and gases, yet strong andnon-immunogenic. For example, for transplantation of islets ofLangerhans, the islets, which have a diameter of 100 to 200 microns,have in the past been encapsulated within microspheres that have adiameter of 400 to 1000 microns. This large diameter can result inslowed diffusion of nutritional molecules and large transplantationvolumes.

In summary, there is a need for materials, and methods of use thereof,which can be used to encapsulate cells and tissues or biologicallyactive molecules which are biocompatible, do not elicit specific ornon-specific immune responses, and which can be polymerized in contactwith living cells or tissue without injuring or killing the cells,within a very short time frame, and in a very thin layer. An importantaspect of the use of these materials in vivo is that they must bepolymerizable within the time of a short surgical procedure or beforethe material to be encapsulated disperses, is damaged or dies.

It is therefore an object of the present invention to provide apolymeric material that can be polymerized in contact with living cellsand tissues, and in a very short time period.

It is a further object of the present invention to provide a polymericmaterial which is biocompatible and resistant to degradation for aspecific time period.

It is a still further object of the present invention to provide apolymeric material which is permeable to nutrients and gases yet canprotect cells and tissues from in vivo attack by other cells.

SUMMARY OF THE INVENTION

Disclosed herein is a method for polymerization of macromers usingvisible or long wavelength ultraviolet light (1 w uv light, 320 nm orgreater) to encapsulate or coat either directly or indirectly livingtissue with polymeric coatings which conform to the surfaces of cells,tissues or carriers thereof under rapid and mild polymerizationconditions. Polymers are formed from non-toxic pre-polymers, referred toherein as macromers, that are water-soluble or substantially watersoluble and too large to diffuse into the cells to be coated. Examplesof macromers include highly biocompatible PEG hydrogels, which can berapidly formed in the presence or absence of oxygen, without use oftoxic polymerization initiators, at room or physiological temperatures,and at physiological pH. Polymerization may be initiated using non-toxicdyes such as methylene blue or eosin Y, which are photopolymerizablewith visible or 1 w uv light. Other dyes that diffuse into the cells butare nontoxic, such as ethyl eosin, may also be used. The process isnon-cytotoxic because little light is absorbed by cells in the absenceof the proper chromophore. Cells are largely transparent to this light,as opposed to short wavelength UV radiation, which is strongly absorbedby cellular proteins and nucleic acids and can be cytotoxic. Low levelsof irradiation (5–500 mW) are usually enough to induce polymerization ina time period of between milliseconds to a few seconds for mostmacromers. A second reason for the lack of cytotoxicity is that thepolymerizable species does not diffuse into cells.

The resulting polymers can act as semipermeable membranes, as adhesivesas tissue supports, as plugs, as barriers to prevent the interaction ofone cell tissue with another cell or tissue, and as carriers forbioactive species. A wide variety of surfaces, with differentgeometries, can be coated with a three dimensionally cross-linkednetwork of these polymeric materials. The polymers can be formed into amatrix for delivery of biologically active materials, includingproteins, polysaccharides, organic compounds with drug activity, andnucleic acids.

In one preferred embodiment, the polymer is used to form a layer on theinside of the lumen of a blood vessel, either for structural support,prevention of thrombosis and inflammatory reactions at the lumensurface, and/or delivery of therapeutic agents to the blood vessel. Inanother preferred embodiment, the polymer is used to create asemipermeable barrier around cells such as islets of Langerhans toprotect the cell by preventing the passage of immunoglobulins moleculesor cells, while allowing free transfer of nutrients, gases and smallcell products. Such treated islets may be useful in treating diseaseswhich result from deficiencies in metabolic processing, or diseases likediabetes which arise from insufficient concentrations of bioregulatormolecules.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a reaction scheme for ethyl eosin initiated polymerization.

FIG. 2 is a schematic of dye-initiated polymerization of a PEG layeraround crosslinked alginate microspheres.

FIG. 3 is a schematic of photopolymerization of a PEG coating onalginate-poly(L-lysine) microspheres suspended in mineral oil.

FIG. 4 is a schematic representation of coextrusion apparatus used formicroencapsulation using laser polymerization.

FIG. 5 is a graph of the number of cells versus gel composition, for theunattached cells obtained from lavage of the peritoneal cavity in micewith different PEO overcoat gel compositions: a—18.5k; b—10% 0.5k, 90%18.5k; c—50% 18.5k, 50% 0.4k; d—10% 0.4k, 90% 35k; e—50% 0.4k, 50% 35k;and f—alginate-poly(L-lysine) control.

FIG. 6 is a graph of the % protein released versus time in minutes, fordiffusion of bovine serum albumin (open squares), human IgG (triangles)and human fibrinogen (closed squares) though a PE0 18.5K-tetraacrylategel.

FIG. 7 is a graph of the % diffusion of bovine serum albumin over timein minutes though PE0 400 diacrylate (open squares) and PEG18.5K-tetracrylate (triangles) gels.

FIG. 8 is a graph of the length in mm of gel produced by argon ion laserinduced polymerization versus log (time) (ms) of trimethylolpropaneusing an amine and ethyl eosin initiation system.

FIGS. 9A, 9B and 9C are creep curves for PEG diacrylate andtetraacrylate gels; test and recovery loads are given below theabscissa: A—1k; B—6K; and C—18.5K PEG gels.

DETAILED DESCRIPTION OF THE INVENTION

As described herein, biocompatible polymeric materials are formed foruse in juxtaposition with biologically active materials or cells andtissue, by free radical polymerization of biocompatible water solublemacromers including at least two polymerizable substituents. Thesepolymeric coating materials can be either homopolymers, copolymers orblock copolymers. As used herein, a polymer is a unit formed having adegree of polymerization greater than 10, and an oligomer has a degreeof polymerization of between 2 and 10, degree of polymerization meaningthe number of repeat units in the structure, e.g., d.p.=3 refers to atrimer. Polymerization of a component that has at least twopolymerizable substituents is equal to gelation; the polymerizationproceeds to form a three-dimensional, cross-linked gel.

Pre-polymers (Macromers) Useful for Making Gels

The general criteria for pre-polymers (referred to herein as macromers)that can be polymerized in contact with biological materials or cellsare that: they are water-soluble or substantially water soluble, theycan be further polymerized or crosslinked by free radicalpolymerization, they are non-toxic and they are too large to diffuseinto cells, i.e., greater than 200 molecular weight. Substantially watersoluble is defined herein as being soluble in a mixture of water andorganic solvent(s), where water makes up the majority of the mixture ofsolvents.

As used herein, the macromers must be photopolymerizable with lightalone or in the presence of an initiator and/or catalyst, such as a freeradical photoinitiator, wherein the light is in the visible or longwavelength ultraviolet range, that is, greater than or equal to 320 nm.Other reactive conditions may be suitable to initiate free radicalpolymerization if they do not adversely affect the viability of theliving tissue to be encapsulated. The macromers must also not generateproducts or heat levels that are toxic to living tissue duringpolymerization. The catalyst or free radical initiator must also not betoxic under the conditions of use.

A wide variety of substantially water soluble polymers exist, some ofwhich are illustrated schematically below. (______) represents asubstantially water soluble region of the polymer, and (=) represents afree radical polymerizable species. Examples include:

Examples of A include PEG diacrylate, from a PEG diol; of B include PEGtriacrylate, formed from a PEG triol; of C include PEG-cyclodextrintetraacrylate, formed by grafting PEG to a cyclodextrin central ring,and further acrylating; of D include PEG tetraacrylate, formed bygrafting two PEG diols to a bis epoxide and further acrylating; of Einclude hyaluronic acid methacrylate, formed by acrylating many sites ona hyaluronic acid chain; of F include PEG-hyaluronic acid-multiacrylate,formed by grafting PEG to hyaluronic acid and further acrylating; of Ginclude PEG-unsaturated diacid ester formed by esterifying a PEG diolwith an unsaturated diacid.

Polysaccharides include, for example, alginate, hyaluronic acid,chondroitin sulfate, dextran, dextran sulfate, heparin, heparin sulfate,heparan sulfate, chitosan, gellan gum, xanthan gum, guar gum, andK-carrageenan. Proteins, for example, include gelatin, collagen, elastinand albumin, whether produced from natural or recombinant sources.

Photopolymerizable substituents preferably include acrylates,diacrylates, oligoacrylates, dimethacrylates, or oligomethoacrylates,and other biologically acceptable photopolymerizable groups.

Synthetic Polymeric Macromers

The water-soluble macromer may be derived from water-soluble polymersincluding, but not limited to, poly(ethylene oxide) (PEO), poly(ethyleneglycol) (PEG), poly(vinyl alcohol) (PVA), poly(vinylpyrrolidone) (PVP),poly(ethyloxazoline) (PEOX) polyaminoacids, pseudopolyamino acids, andpolyethyloxazoline, as well as copolymers of these with each other orother water soluble polymers or water insoluble polymers, provided thatthe conjugate is water soluble. An example of a water soluble conjugateis a block copolymer of polyethylene glycol and polypropylene oxide,commercially available as a Pluronic™ surfactant.

Polysaccharide Macromers

Polysaccharides such as alginate, hyaluronic acid, chondroitin sulfate,dextran, dextran sulfate, heparin, heparin sulfate, heparan sulfate,chitosan, gellan gum, xanthan gum, guar gum, water soluble cellulosederivatives, and carrageenan, which are linked by reaction withhydroxyls or amines on the polysaccharides can also be used to form themacromer solution.

Protein Macromers

Proteins such as gelatin, collagen, elastin, zein, and albumin, whetherproduced from natural or recombinant sources, which are madefree-radical polymerization by the addition of carbon-carbon double ortriple bond-containing moieties, including acrylate, diacrylate,methacrylate, ethacrylate, 2-phenyl acrylate, 2-chloro acrylate, 2-bromoacrylate, itaconate, oliogoacrylate, dimethacrylate, oligomethacrylate,acrylamide, methacrylamide, styrene groups, and other biologicallyacceptable photopolymerizable groups, can also be used to form themacromer solution.

Dye-sensitized Polymerization

Dye-sensitized polymerization is well known in the chemical literature.For example, light from an argon ion laser (514 nm), in the presence ofan xanthin dye and an electron donor, such as triethanolamine, tocatalyze initiation, serves to induce a free radical polymerization ofthe acrylic groups in a reaction mixture (Neckers, et al., (1989) Polym.Materials Sci. Eng., 60:15; Fouassier, et al., (1991) Makromol. Chem.,192:245–260). After absorbing the laser light, the dye is excited to atriplet state. The triplet state reacts with a tertiary amine such asthe triethanolamine, producing a free radical which initiates thepolymerization reaction. Polymerization is extremely rapid and isdependent on the functionality of the macromer and its concentration,light intensity, and the concentration of dye and amine.

Photoinitiating Dyes

Any dye can be used which absorbs light having a frequency between 320nm and 900 nm, can form free radicals, is at least partially watersoluble, and is non-toxic to the biological material at theconcentration used for polymerization. There are a large number ofphotosensitive dyes that can be used to optically initiatepolymerization, such as ethyl eosin, eosin Y, fluorescein,2,2-dimethoxy-2-phenyl acetophenone, 2-methoxy,2-phenylacetophenone,camphorquinone, rose bengal, methylene blue, erythrosin, phloxime,thionine, riboflavin, methylene green, acridine orange, xanthine dye,and thioxanthine dyes.

The preferred initiator dye is ethyle eosin due to its spectralproperties in aqueous solution (absorption max=528 nm, extinctioncoefficient=1.1×10⁵ M^(−t)cm^(−t), fluorescence max=547 nm, quantumyield=0.59). A reaction scheme using ethyl eosin is shown in FIG. 1 asan example. The dye bleaches after illumination and reaction with amineinto a colorless product, allowing further beam penetration into thereaction system.

Cocatalyst

The cacatalysts useful with the photoinitiating dyes are nitrogen basedcompounds capable of stimulating the free radical reaction. Primary,secondary, tertiary or quaternary amines are suitable cocatalysts, asare any nitrogen atom containing electron-rich molecules. Cocatalystsinclude, but are not limited to, triethanolamine, triethylamine,ethanolamine, N-methyl diethanolamine, N,N-dimethyl benzylamine,dibenzyl amine, N-benzyl ethanolamine, N-isopropyl benzylamine,tetramethyl ethylenediamine, potassium persulfate, tetramethylethylenediamine, lysine, ornithine, histidine and arginine.

Examples of the dye/photoinitiator system includes ethyl eosin with anamine, eosin Y with an amine, 2,2-dimethoxy-2-phenoxyacetophenone,2-methoxy-2-phenoxyacetophenone, camphorquinone with an amine, and rosebengal with an amine.

In some cases, the dye may absorb light and initiate polymerization,without any additional initiator such as the amine. In these cases, onlythe dye and the macromer need be present to initiate polymerization uponexposure to light. The generation of free radicals is terminated whenthe laser light is removed. Some photoinitiators, such as2,2-dimethoxy-2-phenylacetophenone, do not require any auxiliary amineto induce photopolymerization; in these cases, only the presence of dye,macromer, and appropriate wavelength light is required.

Means for Polymerization

Photopolymerization

Preferred light sources include various lamps and lasers such as thosedescribed in the following examples, which have a wavelength of about320–800 nm, most preferably about 365 nm or 514 nm.

This light can be provided by any appropriate source able to generatethe desired radiation, such as a mercury lamp, longwave UV lamp, He—Nelaser, or an argon ion laser, or through the use of fiber optics.

Other Means for Polymerization

Means other than light can be used for polymerization. Examples includeinitiation by thermal initiators, which form free radicals at moderatetemperatures, such as benzoyl peroxide, with or without triethanolamine,potassium persulfate, with or without tetramethylethylenediamine, andammonium persulfate with sodium bisulfite.

Incorporation of Biologically Active Materials

The water soluble macromers can be polymerized around biologicallyactive molecules to form a delivery system for the; molecules orpolymerized around cells, tissues, sub-cellular organelles or othersub-cellular components to encapsulate the, biological material. Thewater soluble macromers can also be polymerized to incorporatebiologically active molecules to impart additional properties to thepolymer, such as resistance to bacterial growth or decrease ininflammatory response, as well as to encapsulate tissues. A wide varietyof biologically active material can be encapsulated or incorporated,including proteins, peptides, polysaccharides, organic or inorganicdrugs, nucleic acids, sugars, cells, and tissues.

Examples of cells which can be encapsulated include primary cultures aswell as established cell lines, including transformed cells. Theseinclude but are not limited to pancreatic islet cells, human foreskinfibroblasts, Chinese hamster ovary cells, beta cell insulomas,lymphoblastic leukemia cells, mouse 3T3 fibroblasts, dopamine secretingventral mesencephalon cells, neuroblastoid cells, adrenal medulla cells,and T-cells. As can be seen from this partial list, cells of all types,including dermal, neural, blood, organ, muscle, glandular, reproductive,and immune system cells, as well as species of origin, can beencapsulated successfully by this method. Examples of proteins which canbe encapsulated include hemoglobin, enzymes such as adenosine deaminase,enzyme systems, blood clotting factors, inhibitors or clot dissolvingagents such as streptokinase and tissue plasminogen activator, antigensfor immunization, and hormones, polysaccharides such as heparin,oligonucleotides such as antisense, bacteria and other microbialorganisms, including viruses, vitamins, cofactors, and retroviruses forgene therapy can be encapsulated by these techniques.

The biological material can be first enclosed in a structure such as apolysaccharide gel. (Lim, U.S. Pat. No. 4,352,883; Lim, U.S. Pat. No.4,391,909; Lim, U.S. Pat. No. 4,409,331; Tsang, et al., U.S. Pat. No.4,663,286; Goosen et al., U.S. Pat. No. 4,673,556; Goosen et al., U.S.Pat. No. 4,689,293; Goosen et al., U.S. Pat. No. 4,806,355; Rha et al.,U.S. Pat. No. 4,744,933; Rha et al., U.S. Pat. No. 4,749,620,incorporated herein by reference.) Such gels can provide additionalstructural protection to the material, as well as a secondary level ofperm-selectivity.

Polymerization

The macromers are preferably mixed with initiator, applied to thematerial or site where they are to be polymerized, and exposed toinitiating agent, such as light or heat.

In a preferred method, a photo-initiating system is added to an aqueoussolution of a photopolymerizable macromer to from an aqueous mixture;the biologically active material is added; and the aqueous solutionirradiated with light. The macromer is preferably formed of a watersoluble polymer with photopolymerizable substituents. Light absorptionby the dye/initiator system results in the formation of free radicalswhich initiate polymerization.

In a second preferred method, macromer is coated on the surface of athree-dimensional object which may be of biological origin or asynthetic substrate for implantation in an animal. Water-solublemacromer is mixed with a photoinitiating system to form an aqueousmixture; the mixture is applied to a surface to be coated to form acoated surface; and the coated surface is irradiated with light toinitiate macromer polymerization.

In a variation of this embodiment, the synthetic substrate can be ahydrophilic microsphere, mirocapsule or bead. The hydrophilicmicrospheres are mixed with a water soluble macromer solution incombination with a photoinitiator system to form an aqueous mixture; themicrospheres are suspended with agitation with macromer in oil to forman oil suspension, and the microspheres are irradiated with light.

In another particularly preferred embodiment, a photosensitive dye isabsorbed to a tissue surface which is to be treated, the non-absorbeddye is diluted out or rinsed off the tissue, the macromer solution isapplied to the dye-coupled surface, and polymerization initiated, toresult in interfacial polymerization.

Polymerization can be effected by at least five different methodsutilizing bulk polymerization or interfacial polymerization. Theseembodiments are further described below with respect to specificapplications of the materials and processes for polymerization thereof.

Bulk Polymerization

In bulk polymerization the material to be coated is placed in contactwith a solution of macromer, photoinitiator and optionally cocatalyst,and then polymerization induced, for example, by exposure to radiation.Three examples of bulk polymerization follow:

Bulk Suspension Polymerization Method for Encapsulation of Material

Biological material to be encapsulated is mixed with an aqueous macromersolution, including macromer, cocatalyst and optionally an accelerator,and initiator. Small globular geometric structures such as spheres,ovoids, or oblongs are formed, preferably either by coextrusion of theaqueous solution with air or with a non-miscible substance such as oil,preferably mineral oil, or by agitation of the aqueous phase in contactwith a non-miscible phase such as an oil phase to form small droplets.The macromer in the globules is then polymerized by exposure toradiation. Because the macromer and initiator are confined to theglobules, the structure resulting from polymerization is a capsule inwhich the biological material is enclosed. This is a “suspensionpolymerization” whereby the entire aqueous portion of the globulepolymerizes to form a thick membrane around the cellular material.

Microcapsule Suspension Polymerization Method

In a variation of the bulk suspension method, microencapsulated materialis used as a core about which the macromer is polymerized in asuspension polymerization reaction. The biological material is firstencapsulated within a microsphere, microcapsule, or microparticle(referred to herein collectively as microcapsules), for example, inalginate microcapsules. The microcapsules are then mixed with themacromer solution and initiator, and the macromer solution polymerized.

This method is particularly suitable for use with PEG macromers, takingadvantage of the extreme hydrophilicity of PEG macromers, and isespecially well adapted for use with hydrogel microcapsules such asalginate-poly(L-lysine). The microsphere is swollen in water. When amacromer solution containing catalyst and/or initiator or accelerator isforced to phase separate in a hydrophobic medium, such as mineral oil,the PEG macromer solution prefers to stay on the hydrophilic surface ofthe alginate microcapsule. When this suspension is irradiated, the PEGmacromer undergoes polymerization and gelation, forming a thin layer ofpolymeric, water insoluble gel around the microsphere.

This technique preferably involves coextrusion of the microcapsule in asolution of macromer and initiator, the solution being in contact withair or a liquid which is non-miscible with water, to form droplets whichfall into a solution such as mineral oil in which the droplets are notmiscible. The non-miscible liquid is chosen for its ability to maintaindroplet formation. Additionally, if the membrane-encapsulated materialis to be injected or implanted in an animal, any residue should benon-toxic and non-immunogenic. Mineral oil is a preferred non-miscibleliquid. Once the droplets have contacted the non-miscible liquid, theyare polymerized.

This coextrusion technique results in a crosslinked polymer coat ofgreater than 50 microns thickness. Alternatively, the microcapsules maybe suspended in a solution of macromer and initiator which is agitatedin contact with a non-miscible phase such as an oil phase. The resultingemulsion is polymerized to form a polymer coat, also of greater than 50microns thickness, around the microcapsules.

Bulk Polymerization Method for Tissue Adhesion

The polymeric material can also be used to adhere tissue. A watersoluble polymerizable macromer in combination with a photoinitiator isapplied to a tissue surface to which tissue adhesion is desired; thetissue surface is contacted with the tissue with which adhesion isdesired, forming a tissue junction; and the tissue junction isirradiated with light until the macromers are polymerized. In thepreferred embodiment, this is accomplished in seconds up to minutes,most preferably seconds.

In the preferred embodiment, the macromer mixture is an aqueoussolution, such as that of PEG 400 diacrylate or PEG 18.5K tetraacrylate.When this solution contacts tissue which has a moist layer of mucous orfluid covering it, it intermixes with the moisture on the tissue. Themucous layer on tissue includes water soluble polysaccharides whichintimately contact cellular surfaces. These, in turn, are rich inglycoproteins and proteoglycans. Thus, physical intermixing and forcesof surface interlocking due to penetration into crevices, are some ofthe forces responsible for the adhesion of the PEG gel to a tissuesurface subsequent to crosslinking.

Specific applications for such adhesives may include blood vesselanastomosis, soft tissue reconnection, drainable burn dressings, andretinal reattachment.

Bulk Polymerization to form Tissue Barriers

If the PEG gel is polymerized away from tissue, it then presents a verynon-adhesive surface to cells and tissue in general, due to the highlyhydrophilic nature of the material.

This feature can be exploited to form barriers upon tissues to preventattachment of cells to the coated tissue. Examples of this applicationinclude the formation of barriers upon islets of Langerhans or upon thelumen of blood vessels to prevent thrombosis or vasospasm or vesselcollapse; whether by bulk polymerization (with the polymerizationinitiator mixed in with the macromer) or by interfacial polymerization(with the initiator absorbed to the surface).

Interfacial Polymerization.

For interfacial polymerization, the free radical initiator is adsorbedto the surface of the material to be coated, non-adsorbed initiator isdiluted out or rinsed off, using a rinsing solution or by application ofthe macromer solution, and the macromer solution, optionally containinga cocatalyst, is applied to the material, which is then polymerized. Twoexamples of interfacial polymerization follow:

Microcapsule Interfacial Polymerization Method

Biological material can be encapsulated as described above withreference to suspension polymerization, but utilizing interfacialpolymerization to form the membrane on the surface of the biologicalmaterial or microcapsule. This involves coating the biological materialor-microcapsule with photoinitiator, suspending the biological materialor microcapsules in the macromer solution, and immediately polymerizing,for example, by irradiating. A thin polymer coat, of less than 50microns thickness, is formed around the biological materials or themicrocapsule, because the photoinitiator is present only at themicrocapsule surface and is given insuficient time to diffuse far intothe macromer solution.

In most cases, initiator, such as a dye, will penetrate into theinterior of the biological material or the microcapsule, as well asadsorbing to the surface. When macromer solution, optionally containinga cocatalyst such as triethanolamine, is applied to the surface andexposed to an initatiating agent such as laser light, all the essentialcomponents of the reaction are present only at and just inside theinterface of the biological material or microcapsule and macromersolution. Hence, polymerization and gelation (if multifunctionalmacromer is used), which typically occurs within about 100 msec,initially takes place only at the interface, just beneath it, and justbeyond it. If left for longer periods of time, initiator startsdiffusing from the inner core of the microsphere into the solution;similarly, macromers start diffusing inside the core and a thicker layerof polymer is formed.

Direct Interfacial Polymerization Method.

Interfacial polymerization to form a membrane directly on the surface oftissues. Tissue is directly coated with initiator, excess initiator isremoved, macromer solution is applied to the tissue and polymerized.

Control of Polymer Permeability

The permeability of the coating is determined in part by the molecularweight and crosslinking of the polymer. For example, in the case ofshort PEG chains between crosslinks, the “pore” produced in the networkwill have relatively rigid boundaries and will be relatively small sothat a macromolecule attempting to diffuse through this gel will bepredominantly restricted by a sieving effect. If the chain lengthbetween crosslinks is long, the chain can fold and move around with ahigh motility so that diffusing macromolecules will encounter a freevolume exclusion effect as well as a sieving effect.

Due to these two contrasting effects a straightforward relation betweenmolecular weight cutoff for diffusion and the molecular weight of thestarting oligomer is not completely definable. Yet, a desired releaseprofile for a particular protein or a drug such as a peptide can beaccomplished by adjusting the crosslink density and length of PEGsegments. correspondingly, a desired protein permeability profile can bedesigned to permit the diffusion of nutrients, oxygen, carbon dioxide,waste products, hormones, growth factors, transport proteins, andsecreted cellularly synthesized products such as proteins, whilerestricting the diffusion of immune modulators such as antibodies andcomplement proteins, as well as the ingress of cells, inside the gel, toprotect transplanted cells or tissue. The three dimensional crosslinkedcovalently bonded polymeric network is chemically stable for long-termin vivo applications.

For purposes of encapsulating cells and tissue in a manner whichprevents the passage of antibodies across the membrane but allowspassage of nutrients essential for cellular metabolism, the preferredstarting macromer size is in the range of between 10,000 D and 18,500 D,with the most preferred being around 18,500 D. Smaller macromers resultin polymer membranes of a higher density with smaller pores.

Thickness and Conformation of Polymer Layer

Membrane thickness affects a variety of parameters, includingperm-selectivity, rigidity, and size of the membrane. Thickness can bevaried by selection of the reaction components and/or the reactionconditions. For example, the macromer concentration can be varied from afew percent to 100%, depending upon the macromer. Similarly, moreintense illuminations and longer illuminations will yield thicker filmsthan less intense or shorter illuminations will. Accelerators may alsobe added in varying concentration to control thickness.

For example, N-vinylpyrrolidinone may be added as an accelerator, withhigher concentrations yielding thicker layers than lower concentrations,all other conditions being equal. As an example, N-vinylpyrrolidinoneconcentrations can range from 0 to 0.5%.

In the interfacial polymerization method, the duration of thepolymerization can be varied to adjust the thickness of the polymermembrane formed. This correlation between membrane thickness andduration of irradiation occurs because the photoinitiator diffuses at asteady rate, with diffusion being a continuously occurring process.Thus, the longer the duration of irradiation, the more photoinitiatorwill initiate polymerization in the macromer mix, the more macromer willpolymerize, and the thicker the resulting membrane. Additional factorswhich affect membrane thickness are the number of reactive groups permacromer and the concentration of accelerators in the macromer solution.This technique allows the creation of very thin membranes because thephotoinitiator is first present in a very thin layer at the surface ofthe biological material, and polymerization only occurs where thephotoinitiator is present.

In the suspension polymerization method, a somewhat thicker membrane isformed than with the interfacial polymerization method, since in thesuspension method polymerization occurs throughout the macromersolution. The thickness of membranes formed by the suspension method isdetermined in part by the viscosity of the macromer solution, theconcentration of the macromer in that solution, the fluid mechanicalenvironment of the suspension and surface active agents in thesuspension. These membranes vary in thickness from between 50 and 300microns.

Non-biological Surfaces

The macromer solution and initiator can also be applied to anon-biological surface intended to be placed in contact with abiological environment. Such surfaces include, for example, vasculargrafts, contact lenses, intraocular lenses, ultrafiltration membranes,and containers for biological materials.

It is usually difficult to get good adhesion between polymers of greatlydifferent physicochemical properties. The concept of a surface physicalinterpenetrating network was presented by Desai and Hubbel (N. P. Desaiet al. (1992)). This approach to incorporating into the surface of onepolymer a complete coating of a polymer of considerably differentproperties involved swelling the surface of the polymer to be modified(base polymer) in a mutual solvent, or a swelling solvent, for the basepolymer and for the polymer to be incorporated (penetrant polymer). Thepenetrant polymer diffused into the surface of the base polymer. Thisinterface was stabilized by rapidly precipitating or deswelling thesurface by placing the base polymer in a nonsolvent bath. This resultedin entanglement of the penetrant polymer within the matrix of the basepolymer at its surface in a structure that was called a surface physicalinterpenetrating network.

This approach can be improved upon by photopolymerizing the penetrantpolymer upon the surface of the base polymer in the swollen state. Thisresults in much enhanced stability over that of the previous approachand in the enhancement of biological responses to these materials. Thepenetrant may be chemically modified to be a prepolymer macromer, i.e.capable of being polymerized itself. This polymerization can beinitiated thermally or by exposure to visible, ultraviolet, infrared,gamma ray, or electron beam irradiation, or to plasma conditions. In thecase of the relatively nonspecific gamma ray or electron beam radiationreaction, chemical incorporation-of particularly reactive sites may notbe necessary.

Polyethylene glycol (PEG) is a particularly useful penetrant polymer forbiomedical applications where the lack of cell adhesion is desired. Theprevious work had demonstrated an optimal performance at a molecularweight of 18,500 D without chemical crosslinking. PEG prepolymers can bereadily formed by acrylation of the hydroxyl groups at its termini orelsewhere within the chain. These prepolymers can be readilypolymerized. Photoinitiated polymerization of these prepolymers isparticularly convenient and rapid. There are a variety of visible lightinitiated and ultraviolet light initiated reactions that are initiatedby light absorption by specific photochemically reactive dyes. This sameapproach can be used with other water-soluble polymers, such aspoly(N-vinyl pyrrolidinone), poly(N-isopropyl acrylamide), poly(ethyloxazoline) and many others.

Method for Formation of Polymeric Materials.

Polymeric objects are formed into a desired shape by standard techniquesknown to those skilled in the art, where the macromer solution,preferably containing catalyst and initiator, is shaped, thenpolymerized. For example, slabs may be formed by casting on a flatsurface and discoidal shapes by casting into discoidal containers.Cylinders and tubes can be formed by. extrusion. Spheres can be formedfrom emulsion oil, by co-extrusion with oil, or by co-extrusion withair, another gas or vapor. The macromer is then exposed to conditionssuch as light irradiation, to initiate polymerization. Such irradiationmay occur subsequent to, or, when desired, simultaneously with theshaping procedures.

The macromer may also be shaped in relationship to an internal orexternal supporting structure. Internal-supporting, structures includescreening networks of stable or degradable polymers or nontoxic metals.External structures include, for example, casting the gel within acylinder so that the internal surface of the cylinder is lined with thegel containing the biological materials.

Method for Surface Coating.

These materials can be applied to the treatment of macrocapsularsurfaces, such as those used for ultrafiltration, hemodialysis andnon-microencapsulated immunoisolation of animal tissue. The microcapsulein this case will usually be microporous with a molecular weight cutoffbelow 70,000 Da. It may be in the form of a hollow fiber, a spiralmodule, a flat sheet or other configuration. The surface of such amicrocapsule can be modified using a polymer such as PEG to produce anon-fouling, non-thrombogenic, and non-cell-adhesive. surface. Thecoating serves to enhance biocompatibility and to offer additionalimmunoprotection. Materials which can be modified in this manner includepolysulfones, cellulosic membranes, polycarbonates, polyamides,polyimides, polybenzimidazoles, nylons, poly(acrylonitrile-co-vinylchloride) copolymers, polyurethanes, polystyrene, poly(styreneco-acrylonitriles), poly(vinyl chloride), and poly(ethyleneterephthalate).

A variety of methods can be employed to form biocompatible overcoats,depending on the physical and chemical nature of the surface.Hydrophilic surfaces can be coated by applying a thin layer (forexample, between 50 and 300 microns in thickness) of a polymerizablesolution such as PEG diacrylate containing appropriate amounts of dyeand amine. Hydrophobic surfaces can be first rendered hydrophilic by gasplasma discharge treatment and the resulting surface can then besimilarly coated, or they may simply be treated with a surfactant beforeor during treatment with the PEG diacrylate solution. For example, ahydrophobic polystyrene surface could first be treated by exposure to anO₂ plasma or an N₂ plasma. This results in rendering the surface morehydrophilic by the creation of oxygen-containing or nitrogen containingsurface species, respectively. These species could be further treated byreaction with a substance such as acryloyl chloride, capable ofproducing surface-bound free radical sensitive species. Alteratively, ahydrophobic polystyrene surface could first be treated with asurfactant, such as a poly(ethylene oxide)-poly(propylene oxide) blockcopolymer, which could subsequently be acrylated if desired. Suchtreatments would result in enhanced adhesion between the hydrophiliccoating layers and the hydrophobic material being treated.

Treatment of Textured Materials and Hydrogels.

The surface of materials having a certain degree of surface texture,such as woven dacron, dacron velour, and expandedpoly(tetrafluoro-ethylene) (ePTFE) membranes, can be treated with thehydrogel. Textured and macroporous surfaces allow greater adhesion ofthe PEG gel to the material surface, allowing the coating of relativelyhydrophobic materials such as PTFE and poly(ethylene terephalate) (PET).

Implantable materials such as enzymatic and ion sensitive electrodes,having a hydrogel (such as poly(HEMA), crosslinked poly(vinyl alcohol)and poly(vinyl pyrrolidone)), on their surface, are coated with the morebiocompatible PEO gel in a manner similar to the dye adsorption andpolymerization technique used for the alginate-PLL microspheres in thefollowing examples.

Treatment of Dense Materials.

Gen coatings can be applied to the surfaces of dense (e.g., nontextured,nongel) materials such as polymers, including PET, PTFE, polycarbonates,polyamides, polysulfones, polyurethanes, polyethylene, polypropylene,polystyrene, glass, and ceramics. Hydrophobic surfaces are initiallytreated by a gas plasma discharge or surfactant to render the surfacehydrophilic. This ensures better adhesion of the gel coating to thesurface. Alternatively, coupling agents may be used to increaseadhesion, as readily apparent to those skilled in the art of polymersynthesis and surface modification.

Thin Interfacially Polymerized Coatings Within Blood. Vessels and Uponother Tissues.

The methodology described above can also be used to photopolymerize verythin films of non-degradable polymer coatings with blood vessels toalter the interaction of blood platelets with the vessel wall and todeliver therapeutics such as enzymes and other proteins, polysaccharidessuch as hyaluronic acid, nucleic acids such as antisense and ribozymes,and other organic and inorganic drugs, using the methods describedabove.

The immediate effect of the polymerization of the polymer inside bloodvessels is to reduce the thrombogenicity of an injured blood vesselsurface. This has clear utility in improving the outcome of balloonangioplasty by reducing the thrombogenicity of the vessel and reducingthe incidence of lesions created by balloon dilatation. Another effectof this modification may be to reduce smooth muscle cell hyperplasia.This is expected for two reasons. First, platelets contain a potentgrowth factor, platelet-derived growth factor (PDGF), thought to beinvolved in post-angioplasty hyperplasia. The interruption of thedelivery of PDGF itself poses a pharmacological intervention, in that a“drug” that would have been delivered by the platelets would beprevented from being delivered. Thrombosis results in the generation ofthrombin, which is a known smooth muscle cell mitogen. The interruptionof thrombin generation and delivery to the vessel wall also poses apharmacological intervention. Moreover, there are other growth factorssoluble in plasma which are known to be smooth muscle cell mitogens. Thegel layer presents a permselective barrier on the surface of the tissue,and thus the gel layer is expected to reduce hyperplasia afterangioplasty. Further, the gel may reduce vasospasm by protecting thevessel from exposure to vasoconstrictors such as thrombin and may reducethe incidence of acute reclosure.

The restriction of the polymerization at an interface is a veryimportant advantage. Disease lesions inside a blood vessel are highlyirregular in shape. Thus, it is very difficult to use a preshapedobject, such as a balloon, to make a form which is to contain thepolymerizing material adjacent to the blood vessel.

There are several other organs where one needs to control cellinteraction with tissues or to create similar barriers by bulk orinterfacial polymerization. This methodology is equally applicable tothe other organs, as well as to encapsulation of specific cell types orbiologically active materials such as enzymes for treatment of variousmetabolic defects and diseases, for example, as described below.

(i) Encapsulation of Neurotransmitter-releasing Cells

Paralysis agitans, more commonly called Parkinson's disease, ischaracterized by a lack of the neurotransmitter dopamine within thestriatum of the brain. Dopamine secreting cells such as cells from theventral mesencephalon, from neuroblastoid cell lines or from the adrenalmedulla can be encapsulated using the method and materials describedherein. Cells, including genetically engineered cells, secreting aprecursor for a neurotransmitter, an agonist, a derivative or a mimic ofa particular neurotransmitter or analogs can also be encapsulated.

(ii) Encapsulation of Hemoglobin for Synthetic Erythrocytes

Hemoglobin in its free form can be encapsulated in PEG gels and retainedby selection of a PEG chain length and cross-link density which preventsdiffusion. The diffusion of hemoglobin from the gels may be furtherimpeded by the use of polyhemoglobin, which is a cross-linked form ofhemoglobin. The polyhemoglobin molecule is too large to diffuse from thePEG gel. Suitable encapsulation of either native or crosslinkedhemoglobin may be used to manufacture synthetic erythrocytes. Theentrapment of hemoglobin in small spheres of less than 5 microns indiameter of these highly biocompatible materials would lead to enhancedcirculation times relative to crosslinked hemoglobin or liposomeencapsulated hemoglobin.

(iii) Entrapment of Enzymes for Correction of Metabolic Disorders andChemotherapy

There are many diseases and defects which result from a deficiency inenzymes. For example, congenital deficiency of the enzyme catalasecauses acatalasemia. Immobilization of catalase in PEG gel networkscould provide a method of enzyme replacement to treat this disease.Entrapment of glucosidase can similarly be useful in treating Gaucher'sdisease. Microspherical PEG gels entrapping urease can be used inextracorporeal blood to convert urea into ammonia. Enzymes such asasparaginase can degrade amino acids needed by tumor cells.Immunogenicity of these enzymes prevents direct use for chemotherapy.Entrapment of such enzymes in immunoprotective PEG gels, however, cansupport successful chemotherapy. A suitable formulation can be designedfor either slow release or no release of the enzyme.

(iv) Cellular Microencapsulation for Evaluation of Anti-humanImmunodeficiency Virus Drugs In Vivo

HIV infected or uninfected human T-lymphoblastoid cells can beencapsulated into PEG gels as described for other cells above. Thesemicrocapsules can be implanted in a nonhuman animal and then treatedwith test drugs. After treatment, the microcapsules can be harvested andthe encapsulated cells screened for viability and functional normalcy.Survival of infected T cells indicates successful action of the drug.Lack of biocompatibility is a documented problem in this approach todrug evaluations, but the highly biocompatible gels described hereinshould solve this problem.

(v) Polymerization of Structural Coatings within Blood Vessels and otherTissue Lumens

Just as very thin intravascular coatings can be polymerized within bloodvessels, thicker layers of structural gels may also be polymerizedwithin vessels. These may be used to reduce abrupt reclosure, to holdback vessel wall disections, to resist vasospasm, or to reduce smoothmuscle cell hyperplasia. These gels may be produced by bulk orinterfacial polymerization, and the thicker and higher the crosslinkdensity of the material, the stronger the structure within the vesselwall. This procedure could be carried out upon or within many organs ofthe body.

The following examples are presented to describe preferred embodimentsand utilities of the present invention and are not meant to limit theinvention unless otherwise stated in the claims appended hereto. Takentogether, the examples illustrate representative demonstrations of thebest mode of implementing the invention as currently understood.

EXAMPLE 1 Synthesis of PEG 6K Diacrylate

PEG acrylates of molecular weights 400 Da and 1,000 Da are commerciallyavailable from Sartomer and Dajac Inc., respectively. 20 g of PEG 6Kdiol was dissolved in 200 ml dichloromethane in a 250 ml round bottomflask. The flask was cooled to 0° C. and 1.44 ml of triethyl amine and1.3 ml of acryloyl chloride were added with constant stirring under adry nitrogen atmosphere. The reaction mixture was then brought to roomtemperature and was stirred for 12 hr under a nitrogen atmosphere. Itwas then filtered, and the filtrate was precipitated by adding to alarge excess of hexane. The crude monomer was purified by dissolving indichloromethane and precipitating in hexane. Yield 60%.

EXAMPLE 2 Synthesis of PEG 18.5 K Tetraacrylate

30 g of a tetrahydroxy water soluble PEG (mol wt 18,500) (PEG 18.5k) waspurchased from Polysciences, Inc.

The PEG was dried by dissolving in benzene and distilling off thewater-benzene azeotrope. 59 g of PEG 18.5 k was dissolved in 300 ml ofbenzene in a 500 ml flask. To this, 3.6 ml of triethylamine and 2.2 mlof acryloyl chloride were added under nitrogen atmosphere and thereaction mixture was refluxed for 2 hours. It was then cooled andstirred overnight. The triethyl amine hydrochloride was separated byfiltration and the copolymer was recovered from filtrate byprecipitating in a large excess of hexane. The polymer was furtherpurified by dissolving in methylene chloride and reprecipitating inhexane. The polymer was dried at 50° C. under vacuum for 1 day. Yield68%.

EXAMPLE 3 Coating of Islet-containing Alginate-PLL Microspheres bySurface Dye Adsorption.

The microcapsule interfacial polymerization method was used to formmembrane around alginate-PLL microcapsules containing islets.Alginate-PLL coacervated microspheres, containing one or two humanpancreatic islets each, were suspended in a 1.1% CaCl₂ solution andaspirated free of excess solution to obtain a dense plug ofmicrospheres. A solution of ethyl eosin (0.04% w/v) was prepared in a1.1% CaCl₂ solution. This solution was filter-sterilized by passagethrough a 0.45 μm filter. The plug of microspheres was suspended in 10ml of the eosin solution for 2 min to allow uptake of the dye. Themicrospheres were then washed four times with fresh 1.1% CaCl₂ to removeexcess dye. 2 ml of a solution (23% w/v) of PEG ′18.5 tetraacrylatecontaining 100 μl of a 3.5% w/v solution of triethanolamine inhydroxyethylpiperazine ethanesulfonic acid (HEPES) buffered saline wasadded to 0.5 ml of these microspheres. The microspheres were exposed toargon ion laser light for 30 seconds with periodic agitation. Thesuspension of microspheres was uniformly scanned with the light duringthis period. The microspheres were then washed with calcium solution andthe process was repeated in order to further stabilize the coating.

A static glucose stimulation test (SGS) was performed on isletsencapsulated in the microspheres coated with PEG gel. Data for insulinsecretion in response to this challenge appears in Table 1. The isletsare seen to be viable by dithizone staining. The SGS test data confirmthe vitality and functionality of the islets.

TABLE 1 Function of Encapsulated Islet Cells SGS Glucose Concentration(mg %) Initial Subsequent 60 300 60 Insulin/Islet/hr (μU/ml)* DiffusionOvercoat Method 1.0 10.04 ± 3.56 2.54 ± 0.76 Mineral Oil Overcoat Method1.0 10.23 ± 3.28 1.02 ± 0.78 Free Islet Control 1.0 3.74 ± 1.4  1.9 ±0.17 *Values are mean ± S.D., all are normalized as compared to theinitial 60 mg %, after subjection to the 300 mg % glucose, the isletsare resubjected to the initial dose.

PEG diacrylate macromers can be polymerized identically as the PEGtetraacrylate macromer described in this example.

EXAMPLE 4 Coating Islet-containing Alginate-PLL Microspheres SuspensionPolymerization Method

This method takes advantage of the hydrophilic nature of PEG monomers. 2ml of alginate/PLL microspheres, containing one or two human pancreaticislets each, were mixed with PEG tetraacrylate macromer solution (PEGmol wt 18.5 kD, 23% solution in saline) in a 50 ml transparentcentrifuge tube. Triethanolamine (0.1M) and 0.5 mM ethyl eosin weremixed with macromer solution. The excess macromer solution was decanted,20 ml of mineral oil was added to the tube, and the reaction mixture wasvortexed thoroughly for 5 minutes. Silicone oil will perform equallywell in this synthesis but may have poorer adjuvant characteristics ifthere is any carry-over. Any other water-immiscible liquid may be usedas the “oil” phase. Acceptable triethanolamine concentrations range fromabout 1 mM to about 100 mM. Acceptable ethyl eosin concentrations rangefrom about 0.01 mM to more than 10 mM.

The beads were slightly red due to the thin coating of macromer/dyesolution and were irradiated for 20–50 sec with an argon ion laser(power 50–500 mW). Bleaching of the (red) ethyl eosin color suggestscompletion of the reaction. The beads were then separated from themineral oil and washed several times with saline solution. The entireprocedure was carried out under sterile conditions.

A schematic representation of the macrosphere coating process in oil isshown in FIG. 3. Alginate/polylysine capsules are soluble in sodiumcitrate at pH 12. When these coated microspheres come in contact withsodium citrate at pH 12, the inner alainate/polylysine coacervatedissolves and a PEG polymeric membrane can still be seen (crosslinkedPEG gels are substantially insoluble in all solvents including water andsodium citrate at pH 12). The uncoated control microspheres dissolvecompletely and rapidly in the same solution.

A static glucose challenge was performed on the islets as in Example 3.Data are also shown in Table 1. The islets are viable and functional.

EXAMPLE 5 Encapsulation of Islets of Langerhans

This example makes use of the direct interfacial polymerization. Isletsof Langerhans isolated from a human pancreas were encapsulated in PEGtetraacrylate macromer gels. 500 islets suspended in RPMI 1640 mediumcontaining 10% fetal bovine serum were pelleted by centrifuging at 100 gfor 3 min. The pellet was resuspended in 1 ml of a 23% w/v solution ofPEG 18.5K tetraacrylate macromer in HEPES buffered saline. 5 μl of anethyl eosin solution in vinyl pyrrolidone (at a concentration of 0.5%)was added to this solution along with 100 μl of a 5 M solution oftriethanolamine in saline. 20 ml of a mineral oil was then added to thetube which was vigorously agitated to form a dispersion of droplets200–500 μm in size. This dispersion was then exposed to an argon ionlaser with a power of 250 mW, emitting at 514 nm, for 30 sec. Themineral oil was then separated by allowing the microspheres to settle,and the resulting microspheres were washed twice with phosphate bufferedsaline (PBS), once with hexane and three times with media.

The viability of Islets of Langerhans encapsulated in aPEG-tetraacrylate gel was verified by an acridine orange and propidiumiodide staining method and also by dithizone staining. In order to testfunctional normalcy, a SGS test was performed on these islets. Theresponse of the encapsulated islets was compared to that of free isletsmaintained in culture for the same time period. All islets weremaintained in culture for a week before the SGS was performed. Theresults are summarized in Table 2. It can be seen that the encapsulatedislets secreted significantly (p<0.05) higher amounts of insulin thanthe free islets. The PEG-tetraacrylate gel encapsulation process did notimpair function of the islets and in fact helped them maintain theirfunction in culture better than if they had not been encapsulated.

TABLE 2 Secretion of Insulin from Islet Cells Islet Insulin SecretionGlucose Concentration (mg %) 60 300 60 Insulin/islet/hr (μU/ml)* Freeislets 1.0 3.74 ± 1.40 1.9 ± 0.17 Encapsulated Islets 1.0 20.81 ± 9.36 2.0 ± 0.76 *Values are mean ± S.D., normalized to initial basal level at60 mg % glucose.

EXAMPLE 6 Microencapsulation of Animal Cells

PEG diacrylates of different molecular weight were synthesized by areaction of acryloyl chloride with PEG as in Example 1. A 20 to 30%solution of macromer was mixed with a cell suspension and the ethyleosin and triethanolamine initiating system before exposing it to laserlight through a coextrusion air flow apparatus shown in FIG. 4.Microspheres were prepared by an air atomization process in which astream of macromer was atomized by an annular stream of air. The airflow rate used was 1,600 cc/min and macromer flow rate was 0.5 ml/min.The droplets were allowed to fall into a petri dish containing mineraloil and were exposed to laser light for about 0.15 sec each topolymerize the microspheres and make them insoluble in water. Themicrospheres were separated from the oil and thoroughly washed with PBSbuffer to remove unreacted macromer and residual initiator. The size andshape of the microspheres was dependent on extrusion rate (0.05 to 0.1ml/min) and extruding capillary diameter (18 Ga to 25 Ga). Thepolymerization times were dependent on initiator concentration (ethyleosin concentration (5 μM to 0.5 mM), vinyl pyrrolidone concentration(0.0% to 0.1%), triethanolamine concentration (5 to 100 mM), laser power(10 mW to 1 W), and macromer concentration (greater than 10% w/v).

A PEG diacrylate macromer of molecular weight 400 Da was used as a 30%solution in PBS, containing 0.1 M triethanolamine as a cocatalyst and0.5 mM ethyl eosin as a photoinitiator. The polymerizations were carriedout at physiological pH in the presence of air. This is significantsince radical polymerizations may be affected by the presence of oxygen,and the acrylate polymerization is still rapid enough to proceedeffectively.

The process also works at lower temperatures. For cellularencapsulation, a 23% solution of PEG diacrylate was used with initiatingand polymerization conditions as used in the air atomization technique.Cell viability subsequent to encapsulation was checked with the trypanblue exclusion assay. Human foreskin fibroblasts (HFF), Chinese hamsterovary cells (CHO-Kl), and a beta cell insuloma line (RiN5F) were foundto be viable (more than 95%) after encapsulation. A wide range of PEGdiacrylate concentrations greater than 10% can be used equallyeffectively, as can PEG tetraacrylate macromers.

EXAMPLE 7 Coating of Animal Cell-containing Alginate-PLL Microspheresand Individual Cells by Surface Dye Adsorption

Alginate-PLL coacervated microspheres containing animal cells weresuspended in a 1.1% CaCl₂ solution and were aspirated free of excesssolution to obtain a dense plug of microspheres. The plug ofmicrospheres was suspended in 10 ml of eosin solution for 2 min to allowdye uptake. 2 ml of a solution (23% w/v) of PEG 18.5 tetraacrylatecontaining 100 μl of a 3.5 w/v solution of triethanolamine in HEPESbuffered saline was added to 0.5 ml of these microspheres. Themicrospheres were exposed to an argon ion laser for 30 seconds withperiodic agitation. The suspension of microspheres was uniformly scannedwith the laser during this period. The microspheres were then washedwith calcium solution and the process was repeated once more in order toattain a stable coating.

In order to verify survival of cells after the overcoat process, cellsin suspension without the alginate/PLL microcapsule were exposed tosimilar polymerization conditions. 1 ml of lymphoblastic leukemia cells(RAJI) (5×10⁵ cells) was centrifuged at 300 g for 3 min. 1 ml of a 0.04%filter sterilized ethyl eosin solution was phosphate buffered saline(PBS) is added and the pellet was resuspended. The cells were exposed tothe dye for 1 min and washed twice with PBS and then pelleted. 10 μl oftriethanolamine solution (0.1 M) was added to the pellet and the tubewas vortexed to resuspend the cells. 0.5 ml of PEG 18.5K tetraacrylatemacromer was then mixed into this suspension and the resulting mixtureexposed to an argon ion laser (514 nm, 50 mW) for 45 sec. The cells werethen washed twice with 10 ml saline and once with media (RPMI 1640 with10% FCS and 1% antibiotic, antimycotic). A thin membrane ofPEG-tetraacrylate gel was observed forming around each individual cell.

No significant difference in viability was seen between the controlpopulation (93% viable) and the treated cells (95% viable) by trypanblue exclusion. An assay for cell viability and function was performedby adapting the MTT-Formazan assay for the RAJI cells. This assayindicates greater than 90% survival. Similar assays were performed withtwo other model cell lines. Chinese hamster ovary cells (CHO-Kl) show nosignificant difference (p<0.05) in metabolic function as evaluated bythe MTT-Formazan assay. 3T3 mouse fibroblasts also show no significantreduction (p>0.50) in metabolic activity.

EXAMPLE 8 Coating Animal Cell Containing Alginate-PLL MicrosphereSuspension Polymerization Method.

Using the method described in Example 4, RAJI cells encapsulated inalginate-PLL microspheres were coated with a PEG 18.5K tetraacrylatepolymeric membrane. Viability of these cells was checked by trypan blueexclusion and found to be more than 95% viable.

EXAMPLE 9 Coating of Individual Islets of Langerhans by Surface DyeAdsorption

Using the method described in Example 7, ethyl eosin was adsorbed to thesurfaces of islets, a solution of the PEG macromer with triethanolaminewas applied to the dye-coated cells, and the cells were exposed to lightfrom an argon-ion laser to form a thick PEG polymeric membrane on thesurface of the islets. Islet viability was demonstrated by lack ofstaining with propidium iodide.

EXAMPLE 10 Biocompatibility of PEG on Microspheres

In vivo evaluation of the extent of inflammatory response tomicrospheres prepared in Examples 7 and 8 was carried out byimplantation in the peritoneal cavity of mice. Approximately 0.5 ml ofmicrospheres were suspended in 5 ml of sterile HEPES buffered saline.2.5 ml of this suspension was injected into the peritoneal cavity of ICRmale swiss white mice. The microspheres were recovered after 4 days bylavage of the peritoneal cavity with 5 ml of 10U heparin/ml PBS. Theextent of cellular growth on the microspheres was visually inspectedunder a phase contrast microscope. The number of unattached cellspresent in the recovered lavage fluid was counted using a Coultercounter.

Photographs were taken of alginate-poly(L-lysine) microspheres recoveredafter 4 days, and similar spheres which had been coated with PEG gel bythe dye diffusion process before implantation. As expected, bilayeralginate-polylysine capsules not containing an outer alginate layer werecompletely covered with cells due to the highly cell adhesive nature ofthe PLL surface, whereas the PEG coated microspheres were virtually freeof adherent cells. Almost complete coverage of alginate-poly (L-lysine)was expected because polylysine has amino groups on the surface, andpositively charged surface amines can interact with cell surfaceproteoglycans and support cell growth (Reuveny, et al., (1983)Biotechnol., Bioeng., 25:469–480). The photographs strongly indicatethat the highly charged and cell adhesive surface of PLL is covered by astable layer of PEG gel. The integrity of the gel did not appear to becompromised.

The non-cell-adhesive tendency of these microspheres was evaluated as apercentage of the total microsphere area which appears covered withcellular overgrowth. These results are summarized in Table 3.

TABLE 3 Microsphere Coverage with Cell Overgrowth Following ImplantationIntraperitoneally for 4 Days. Composition of PEG gel % Cell coverage18.5K <1 18.5k 90%:0.4k 10% <1 18.5k 50%:0.4k 50% <1  35k 90%:0.4k 10%5–7  35k 50%:0.4k 50% <1 Alginate-poly(L-lysine) 60–80

An increase in cell count is a result of activation of residentmacrophages which secrete chemical factors such as interleukins andinduce nonresident macrophages to migrate to the implant site. Thefactors also attract fibroblasts responsible for collagen synthesis. Thevariation of cell counts with chemical composition of the overcoat isshown in FIG. 5( a–f). It can be seen from the figure that all PEGcoated spheres have substantially reduced cell counts. This isconsistent with the PEG overcoat generally causing no irritation of theperitoneal cavity.

However, PEG composition does make a difference in biocompatibility, andincreasing molecular weights are associated with a reduction in cellcounts. This could be due to the gels made from higher molecular weightoligomers having higher potential for steric repulsion due to the longerchain lengths.

EXAMPLE 11 Permeability of PEG Gels

20 mg of bovine serum albumin, human IgG, or human fibrinogen wasdissolved in 2 ml of a 23% w/v solution of oligomeric PEG 18.5ktetraacrylate in PBS. This solution was laser polymerized to produce agel 2 cm ×2 cm×0.5 cm in size. The diffusion of bovine serum albumin,human IgG and human fibrinogen (mol wt 66 kDa, 150 kDa and 350 kDarespectively) was monitored through the 2 cm×2 cm face of these gelsusing a total protein assay reagent (Biorad). A typical release profilefor PEG 18.5K gel was shown in FIG. 6. This gel allowed a slow transportof albumin but did not allow IgG and fibrinogen to diffuse. Thisindicates that these gels are capable of being used as immunoprotectivebarriers. This is a vital requirement for a successful animal tissuemicroencapsulation material.

The release profile was found to be a function of crosslink density andmolecular weight of the polyethylene glycol segment of the monomer. FIG.7 shows the release of bovine serum albumin (BSA) through gels made from23% solutions of PEO diacrylates and tetraacrylates of 0.4K and 18.5K,respectively. It is evident that the 18.5K gel was freely permeable toalbumin while the 0.4K gel restricted the diffusion of albumin. Therelease of any substance from these gels depends on the crosslinkdensity of the network and also depends on the motility of the PEGsegments in the network. This effect was also dependent upon thefunctionality of the macromer. For example, the permeability of a PEG18.5K tetraacrylate gel was less than that of an otherwise similar PEG20K diacrylate gel.

EXAMPLE 12 Treatment of Silicone Rubber to Form PEG Gel Layer to EnhanceBiocompatibility

2×2 cm pieces of medical grade silicone rubber were soaked for 1 h inbenzene containing 23% 0.4k PEG diacrylate and 0.5%2,2-dimethoxy-2-phenyl acetophenone. The swollen rubber was irradiatedfor 15 min with a long wave UV lamp (365 nm). After irradiation, thesample was rinsed in benzene and dried. The air contact angles ofsilicone rubber under water were measured before and after treatment.The decreased contact angle of 50° after treatment, over the initialcontact angle of 63° for untreated silicone rubber, reflects anincreased hydrophilicity due to the presence of the PEG gel on therubber surface.

This technique demonstrates that macromer polymerization can be used tomodify a polymer surface so as to enhance biocompatibility. Forinstance, a polyurethane catheter can be treated by this method toobtain an implantable device coated with PEG. The PEG was firmlyanchored anchored to the surface of the polyurethane catheter becausethe macromer was allowed to penetrate the catheter surface (to a depthof 1–2 microns) during the soaking period before photopolymerization.Upon irradiation, an interpenetrating network of PEG and polyurethaneresults. The PEG was thereby inextricably intertwined with thepolyurethane.

EXAMPLE 13 Rate of Polymerization

The kinetics of a typical reaction were determined to demonstraterapidity of gelation in laser-initiated polymerizations ofmultifunctional acrylic monomers. Trimethylolpropyl triacrylate,containing 5×10⁴M ethyl eosin as a photoinitiator in 10 μmoles ofN-vinyl pyrrolidone per ml of macromer mix and 0.1 M of triethanolamineas a cocatalyst, was irradiated with a 500 mW argon ion laser (514 nmwavelength, power 3.05×10⁵ W/m², beam diameter 1 mm, average geldiameter produced 1 mm). A plot of the length of the spike of gel formedby penetration of the laser beam into the gel versus laser irradiationtime is shown in FIG. 8.

100 μl of a 23% w/w solution of various macromers in HEPES bufferedsaline containing 3 μl of initiator solution (300 mg/ml of 2,2dimethoxy-2-phenoxyacetophenone in N-vinyl pyrrolidone) was placed on aglass coverslip and irradiated with a low intensity long wave UV (LWUV)lamp (Black-Ray, model 3-100A with flood). The times required forgelation to occur were noted and are given in Table 4. These times aretypically in the range of 10 seconds.

TABLE 4 Gelling Times of Irradiated Polymer Gel Time (sec) Polymer Size(mean ± S.D.) 0.4K 6.9 ± 0.5   1K 21.3 ± 2.4    6K 14.2 ± 0.5   10K 8.3± 0.2 18.5K  6.9 ± 0.1  20K 9.0 ± 0.4

Time periods of about 10–100 ms are sufficient to gel a 300 μm diameterdroplet, a typical size of gel used in microencapsulation technology.This rapid gelation, if used in conjunction with proper choice ofmacromers, can lead to entrapment of living cells in a three dimensionalcovalently bonded polymeric network. Monochromatic laser light will notbe absorbed by the cells unless a proper chromophore is present, and isconsidered to be harmless if the wavelength is more than about 400 nm.Exposure to long wavelength ultraviolet light, greater than 360 nm, isharmless at practical intensities and durations.

The polymerization rate of the macromer will depend on the macromerconcentration, the initiator concentration, and the functionality of themacromer, e.g., the difunctionality of a PEG diacrylate or thetetrafunctionality of a PEG tetraacrylate, as well as the degree ofacrylation of the material.

EXAMPLE 14 PEG Gel Interactions

Biocompatibility with HFF (human foreskin fibroblasts) cells wasdemonstrated as follows. HFF cells were seeded on PEG 18.5Ktetraacrylate gels at a density of 18,000 cells/cm² in Dulbecco'smodification of Eagle's medium containing 10% fetal calf serum. The gelswere then incubated at 37° C. in a 5% CO₂ environment for 4 hr. At theend of this time the gels were washed with PBS to remove anynon-adherent cells and were observed under a phase contrast microscopeat a magnification of 200×.

The growth of these cells on a typical PEG gel was compared to thegrowth of these cells on a glass surface. The number of attachedcells/cm² was found to be 510±170 on the gel surfaces as compared to13,200±3,910 for a control glass surface. The cells on these gelsappeared rounded and were not in their normal spread morphology,strongly indicating that these gels do not encourage cell attachment.

Biocompatibility on microspheres was demonstrated as follows. Aphotograph of microspheres explanted from mice as in Example 10 wastaken; after 4 days very little fibrous overgrowth is seen. Theresistance of PEG chains to protein adsorption and hence cellular growthis well documented. Table 5 summarizes the extent of cellular overgrowthseen on these microspheres formed of various PEG diacrylate gels afterimplanted intraperitoneally for four days.

TABLE 5 Extent of Cellular Overgrowth on Gels PEG Diacrylate for GelsExtent of Cellular Overgrowth (mol wt, Daltons) 400  5–10% 1,000 15–25%5,000  3–5% 6,000  2–15% 10,000 10–20% 18,500  4–10%

EXAMPLE 15 Characterization and Mechanical Analysis of PEG Gels

10 μl of an initiator solution containing 30 mg/ml of2,2-dimethoxy-2-phenyl acetophenone in vinyl-2-pyrrolidone was added perml to 23% w/v solutions of PEG diacrylates (0.4K, 6K, 10K) and PEGtetraacrylates (18.5K). The solution of initiator containing macromerwas placed in a 4.0×1.0×0.5 cm mold and exposed to a long waveultraviolet lamp (365 nm) for approximately 10 seconds to inducegelation. Samples were allowed to equilibrate in phosphate bufferedsaline (pH 7.4) for 1 week before analysis was performed.

A series of “dogbone” samples (samples cut from a slab into the shape ofa dogbone, with wide regions at both ends and a narrower long region inthe middle) were cut from ultimate tensile strength tests. Thickness ofthe samples was defined by the thickness of the sample from which theyare cut. These thicknesses ranged from approximately 0.5 mm to 1.75 mm.The samples were 20 mm long and 2 mm wide at a narrow “neck” region. Thestress strain tests were run in length control at a rate of 4% persecond. After each test, the cross sectional area was determined. Table6 shows the ultimate tensile strength data. It is seen that the lowermolecular weight macromers in general give stronger gels which are lessextensible than those made using the higher molecular weight macromers.The PEG 18.5K tetraacrylate gel is seen to be anomalous in this series,resulting from the multifunctionality of the macromer and thecorresponding higher crosslinking density in the resulting gel. Thistype of strengthening result could be similarly obtained with macromersobtained having other than four free radical-sensitive groups, such asacrylate groups.

TABLE 6 Gel Strength Tests PEG Acrylate Precursor Molecular Weight 0.4K6K 10K 18.5K Stress 168 +/− 51  98 +/− 15 33 +/− 7  115 +/− 56  (kPa)* %8 +/− 3 71 +/− 13 110 +/− 9  40 +/− 15 Strain* Slope* 22 +/− 5  1.32 +/−0.31 0.27 +/− 0.04 2.67 +/− 0.55 *Values are mean +/− S.D.

For the creep tests, eight samples approximately 0.2×0.4×2 cm wereloaded while submersed in saline solution. They were tested with aconstant unique predetermined load for one hour and a small recoveryload for ten minutes. Gels made from PEG diacrylates of 1K, 6K, and 10K,and PEG tetraacrylates of 18.5K PEG molecular weight were used for thisstudy. The 10K test was terminated due to a limit error (the samplestretched beyond the travel of the loading frame). The 1K sample wastested with a load of 10 g and a recovery load of 0.2 g. The 6K samplewas tested at a load of 13 g with a recovery load of 0.5 g. The 18.5Ksample was tested at a load of 13 g with a recovery load of 0.2 g. Thechoice of loads for these samples produced classical creep curves withprimary and secondary regions. The traces for creep for the 1K, 6K, and18.5K samples appear in FIGS. 9A, 9B, and 9C, respectively.

EXAMPLE 16 Water Content of PEG Gels

Solutions of various macromers were made as described above. Gels in theshape of discs were made using a mold. 400 μl of solution was used foreach disc. The solutions were irradiated for 2 minutes to ensurethorough gelation. The disc shaped gels were removed and dried undervacuum at 60° C. for 2 days. The discs were weighed (W1) and thenextracted repeatedly with chloroform for 1 day. The discs were driedagain and weighed (W2). The gel fraction was calculated as W2/W1. Thisdata appears in Table 7.

Subsequent to extraction, the discs were allowed to equilibrate with HBSfor 6 hours and weighed (W3) after excess water had been carefullyswabbed away. The total water content was calculated as (W3−W2)×100/W3.The data for gel water contents is summarized in the following table.

TABLE 7 Data for gel water contents. Polymer Code % Total Water % GelContent 0.4 — 99.8 ± 1.9 1K 79.8 ± 2.1 94.5 ± 2.0 6k 95.2 ± 2.5 69.4 ±0.6 10k  91.4 ± 1.6 96.9 ± 1.5 18.5k   91.4 ± 0.9 80.3 ± 0.9 20k  94.4 ±0.6 85.0 ± 0.4

EXAMPLE 17 Mechanical Stability of PEG Gels after Implantation

PEG diacrylate (10K) and PEG tetraacrylate (mol, wt. 18.5 k) were castin dogbone shapes as described in Example 15. 23% w/w PEG-diacrylate ortetraacrylate in sterile HEPES buffered saline (HBS) (0.9% NaCl, 10 mMHEPES, pH 7.4), containing 900 ppm of2,2-dimethoxy-2-phenoxyacetophenone as initiator, was poured into analuminum mold and irradiated with a LWUV lamp (Black ray) for 1 min. Theinitial weights of these samples were found after oven-drying these gelsto constant weight. The samples were Soxhlet-extracted with methylenechloride for 36 hours in order to leach out any unreacted prepolymerfrom the gel matrix (sol-leaching), prior to testing. The process ofextraction was continued until the dried gels gave constant weight.

ICR Swiss male white mice, 6–8 weeks old (Sprague-Dawley), wereanesthetized by an intraperitoneal injection of sodium pentobarbital.The abdominal region of the mouse was shaved and prepared with betadine.A ventral midline incision 10–15 mm long was made. The polymer sample,fully hydrated in sterile PBS (Phosphate buffered saline) or HEPESbuffered saline (for calcification studies), was inserted through theincision and placed over the mesentery, away from the wound site. Theperitoneal wall was closed with a lock stitched running suture (4.0silk, Ethicon). The skin was closed with stainless steel skin staples,and a topical antibiotic (Furacin) was applied over the incision site.Three animals were used for each time point. One dogbone sample wasimplanted per mouse and explanted at the end of 1 week, 3 weeks, 6weeks, and 8 weeks. Explanted gels were rinsed in HBS twice and thentreated with 0.3 mg/ml pronase (Calbiochem) to remove any adherent cellsand tissue. The samples were then oven-dried to a constant weightextracted and reswelled as mentioned before.

Tensile stress strain test was conducted on both control (unimplanted)and explanted dogbones in a small horizontal Instron-like device. Thedevice is an aluminum platform consisting of two clamps mounted flat ona wooden board between two parallel aluminum guide. The top clamp isstationary while the bottom clamp is movable. Both the frictionalsurfaces of the moving clamp and the platform are coated with aluminumbacked Teflon (Cole-Parmer) to minimize frictional resistance. Themoving clamp is fastened to a device capable of applying a graduallyincreasing load. The whole set-up is placed horizontally under adissecting microscope (Reichert) and the sample elongation is monitoredusing-a video camera. The image from the camera is acquired by an imageprocessor (Argus-10, Hamamatsu) and sent to a monitor. After breakage, across section of the break surface is cut and the area measured- Theload at break is divided by this cross section to find the maximumtensile stress. Table 8 lists the stress at fracture of PEGtetraacrylate (18.5K) hydrogels explanted at various time intervals. Nosignificant change in tensile strength is evident with time. Thus, thegels appear mechanically stable to biodegradation in vivo within themaximum time frame of implant in mice.

TABLE 8 Resistance to degradation of Polymer Implants TIME STRESS (KPa)STRAIN AVE. IMPLANTED (mean ± error*) (mean ± error*) 1 WK 52.8 ± 16.70.32 ± 0.19 3 WK 36.7 ± 10.6 0.37 ± 0.17 6 WK 73.3 ± 34.9 0.42 ± 0.26 8WK 34.1# 0.30# CONTROL 44.9 ± 5.3  0.22 ± 0.22 *Error based on 90%confidence limits. #Single sample.

EXAMPLE 18 Monitoring of Calcification of PEG Gels

Disc shaped PEG-tetraacrylate hydrogels (mol. wt. 18.5 k) were implantedintraperitoneally in mice as described above for a period of 1 week, 3weeks, 6 weeks or 8 weeks. Explanted gels were rinsed in HBS twice andtreated with Pronase (Calbichem) to remove cells and cell debris. Thesamples were then equilibrated in HBS to let free Ca⁺⁺ diffuse out fromthe, gel matrix. The gels were then oven-dried (Blue-M) to a constantweight and transferred to Aluminum oxide crucibles (COORS, hightemperature resistant). They were incinerated in a furnace at 700° C.for at least 16 hours. Crucibles were checked for total incineration, ifany residual remnants or debris was seen they were additionallyincinerated for 12 hours. Subsequently, the crucibles were filled with 2ml of 0.5 M HCl to dissolve Ca⁺⁺ salt and other minerals in the sample.This solution was filtered and analyzed with atomic absorptionspectroscopy (AA) for calcium content.

Calcification data on PEG-tetraacrylate (mol. wt. 18.5K) gel implants isgiven in Table 9. No significant increase in calcification was observedup to an 8 week period of implantation in mice.

TABLE 9 Calcification data on PEG-tetraacrylate (mol. wt. 18.5K) gelimplants TIME CALCIFICATION (mean ± error*) (Days) (mg Calcium/g of Drygel wt.) 7 2.33 ± 0.20 21  0.88 ± 0.009 42 1.08 ± 0.30 56 1.17 ± 0.26*Error based on 90% confidence limits.

EXAMPLES 19 Use of PEG Gels as Adhesive to Rejoin Severed Nerve

A formulation of PEG tetraacrylate (10%, 18.5K), was used as adhesivefor stabilizing the sutureless apposition of the ends of transectedsciatic nerves in the rat. Rats were under pentobarbital anesthesiaduring sterile surgical procedures. The sciatic nerve was exposedthrough a lateral approach by deflecting the heads of the bicepsfemoralis at the mid-thigh level. The sciatic nerve was mobilized forapproximately 1 cm and transected with iridectomy scissors approximately3 mm proximal to the tibial-peroneal bifurcation. The gap between theends of the severed nerves was 2–3 mm. The wound was irrigated withsaline and lightly swabbed to remove excess saline. Sterile,unpolymerized PEG tetraacrylate solution was applied to the wound, Usingdelicate forceps to hold the adventitia or perineurium, the nerve endswere brought into apposition, the macromer solution containing2,2-dimethoxy-2-phenoxyacetophenone as a photoinitiator applied to thenerve ends and the wound was exposed to long wavelength UV-light (365nm) for about 10 sec to polymerize the adhesive. The forceps were gentlypulled away. Care was taken to prevent the macromer solution fromflowing between the two nerve stumps. Alternatively, the nerve stumpjunction was shielded from illumination, e.g., with a metal foil, toprevent gelation of the macromer solution between the stumps; theremaining macromer solution was then simply washed away.

In an alternative approach, both ends of the transected nerve can beheld together with one pair of forceps. Forceps tips are coated lightlywith petrolatum to prevent reaction with the adhesive.

The polymerized adhesive serves to encapsulate the wound and adhere thenerve to the underlying muscle. The anastomosis of the nerve endsresists gentle mobilization of the joint, demonstrating a moderatedegree of stabilization. The muscle and skin were closed with sutures.Re-examination after one month shows that severed nerves remainreconnected, despite unrestrained activity of the animals.

EXAMPLE 20 Surgical Adhesive

Abdominal muscle flaps from female New Zealand white rabbits wereexcised and cut into strips 1 cm×5 cm. The flaps were approximately 0.5to 0.8 cm thick. The lap joint, 1 cm×1 cm, was made using two suchflaps. Two different PEO di- and tetra-acrylate macromer compositions,0.4K (di-) and 18.5K (tetra-), were evaluated. The 0.4K composition wasa viscous liquid and was used without further dilution. The 18.5Kcomposition was used as a 23% w/w solution in HBS. 125 μl of ethyl eosinsolution in n-vinyl pyrrolidone (20 mg/ml) along with 50 μl oftriethanolamine was added to each ml of the adhesive solution. 100 μl ofadhesive solution was applied to each of the overlapping flaps. The lapjoint was then irradiated by scanning with a 2 W argon ion laser for 30seconds from each side. The strength of the resulting joints wasevaluated by measuring the force required to shear the lap joint. Oneend of the lap joint was clamped and an increasing load was applied tothe other end, while holding the joint horizontally until it failed.Four joints were tested for each composition. The 0.4K joints had astrength of 12.0±6.9 KPa (mean ± S.D.), while the 18.5K joints had astrength of 2.7±0.5 KPa. It is significant to note that it was possibleto achieve photopolymerization and reasonable joint strength despite the6–8 mm thickness of tissue. A spectrophotometric estimate using 514 nmlight showed less than 1% transmission through such muscle tissue.

EXAMPLE 21 Modification of Polyvinyl Alcohol

2 g of polyvinyl alcohol (mol wt 100,000–110,000) was dissolved in 20 mlof hot DMSO. The solution was cooled to rooms temperature and 0.2 ml oftriethylamine and 0.2 ml of acryloyl chloride was added with vigorousstirring, under an argon atmosphere. The reaction mixture was heated to70° C. for 2 hr and cooled. The polymer was precipitated in acetone,redissolved in hot water and precipitated again in acetone. Finally itwas dried under vacuum for 12 hr at 60° C. 5–10% w/v solution of thispolymer in PBS was mixed with the UV photoinitiator and polymerizedusing long wavelength UV light to make microspheres 200–1,000 microns insize.

These microspheres were stable to autoclaving in water, which indicatesthat the gel is covalently cross-linked. The gel is extremely elastic.This macromer, PVA multiacrylate, may be used to increase thecrosslinking density in PEG diacrylate gels, with corresponding changesin mechanical and permeability properties. This approach could bepursued with any number of water-soluble polymers chemically modifiedwith photopolymerizable groups, for example with water-soluble polymerschosen from polyvinylpyrrolidone, polyethyloxazoline,polyethyleneoxide-polypropyleneoxide copolymers, polysaccharides such asdextran, alginate, hyaluronic acid, chondroitin sulfate, heparin,heparin sulfate, heparan sulfate, guar gum, gellan gum, xanthan gum,carrageenan gum, and proteins, such as albumin, collagen, and gelatin.

EXAMPLE 22 Use of Alternative Photopolymerizable Moieties

Many photopolymerizable groups may be used to enable gelation. Toillustrate a typical alternative synthesis, a synthesis for PEG 1Kurethane methacrylate is described as follows:

In a 250 ml round bottom flask, 10 g of PEG 1K diol was dissolved in 150ml benzene. 3.38 g of 2-isocyanatoethylmethacrylate and 20 μl ofdibutyltindilaurate were slowly introduced into the flask. The reactionwas refluxed for 6 hours, cooled and poured into 1000 ml hexane. Theprecipitate was then filtered and dried under vacuum at 60° C. for 24hours. In this case, a methacrylate free radical polymerizable group wasattached to the polymer via a urethane linkage, rather than an esterlink as is obtained, e.g. when reacting with aryloxyl chloride.

EXAMPLE 23 Formation of Alginate-PLL-alginate Microcapsules withPhotopolymerizable Polycations

Alginate-polylysine-alginate microcapsules are made by adsorbing, orcoacervating, a polycation, such as polylysine (PLL), upon a gelledmicrosphere of alginate. The resulting membrane is held together bycharge-charge interactions and thus has limited stability. To increasethis stability, the polycation can be made photopolymerizable by theaddition of a carbon-carbon double bond, for example. This can be usedto increase the stability of the membrane by itself, or to react, forexample, with photopolymerizable PEG to enhance biocompatibility.

To illustrate the synthesis of such a photopolymerizable polycation, 1 gof polyallyiamine hydrochloride was weighed in 100 ml glass beaker anddissolved in 10 ml distilled water (DW). The pH of the polymer solutionwas adjusted to 7 using 0.2 M sodium hydroxide solution. The polymer wasthen separated by precipitating in a large excess of acetone. It wasthen redissolved in 10 ml DW and the solution was transferred to 50 mlround bottom flask. 0.2 ml glycidyl methacrylate was slowly introducedinto the reaction flask and the reaction mixture was stirred for 48hours at room temperature. The solution was poured into 200 ml acetoneand the precipitate was separated by filtration and dried in vacuum.This macromer is useful in photochemically stabilizing analginate-PLL-alginate, both in the presence or in the absence of asecond polymerizable species such as a PEG diacrylate.

In addition to use in encapsulating cells in materials such as alginate,such photopolymerizable polycations may be useful as a primer orcoupling agent to increase polymer adhesion to cells, cell aggregates,tissues and synthetic materials, by virtue of adsorption of thephotopolymerizable polymer bonding to the PEG photopolymerizable gel.

EXAMPLE 24 Encapsulation of Hemoglobin for Synthetic Erythrocytes

Hemoglobin in its free form can be encapsulated in PEG gels and retainedby selection of a PEG chain length and cross-link density which preventsdiffusion. The diffusion of hemoglobin from the gels may be furtherimpeded by the use of polyhemoglobin, which is a cross-linked form ofhemoglobin. The polyhemoglobin molecule is too large to diffuse from thePEG gel. Suitable encapsulation of either native or crosslinkedhemoglobin can be used to manufacture synthetic erythrocytes. Theentrapment of hemoglobin in small spheres, less than five microns, ofthese highly biocompatible materials should lead to enhanced circulationtimes relative to crosslinked hemoglobin or liposome encapsulatedhemoglobin.

Hemoglobin in PBS is mixed with the prepolymer in the followingformulation:

Hemoglobin at the desired amount

PEG DA (MW 10000) 35%

PEG DA (MW 1000) 5%

PBS 60%

with 2,2-dimethoxy, 2-phenyl acetophenone at 1.6% of the above solution.

This solution is placed in mineral oil at a ratio of 1 parthemoglobin/prepolymer solution to 5 parts mineral oil and is rapidlyagitated with a motorized mixer to form an emulsion, which is thenilluminated with a long-wavelength ultraviolet light (360 nm) for 5 minto crosslink the PEG prepolymer to form a gel. The molecular weight ofthe prepolymer can be selected to resist the diffusion of the hemoglobinfrom the gel, with smaller PEG molecular weights giving less diffusion.PEG DA of molecular weight 100,000, further crosslinked with PEG DA1000, should possess the appropriate permselectivity to restricthemoglobin diffusion, and it should possess the appropriatebiocompatibility to circulate within the bloodstream.

EXAMPLE 25 Entrapment of Enzymes for Correction of Metabolic Disordersand Chemotherapy

Congenital deficiency of the enzyme catalase causes acatalasemia.Immobilization of catalase in PEG gel networks could provide a method ofenzyme replacement to treat this disease. Entrapment of glucosidase cansimilarly be useful in treating Gaucher's disease. Microspherical PEGgels entrapping urease can be used in extracorporeal blood to converturea into ammonia. Enzymes such as asparaginase can degrade amino acidsneeded by tumor cells. Immunogenicity of these enzymes prevents directuse for chemotherapy. Entrapment of such enzymes in PEG gels, however,can support successful chemotherapy. A suitable formulation can bedeveloped for either slow release or no release of the enzyme.

Catalase in PBS is mixed with the prepolymer in the followingformulation:

Catalase at the desired amount

PEG DA (MW 10000) 35%

PEG DA (MW 1000) 5%

PBS 60%

with 2,2-dimethoxy, 2-phenyl acetophenone at 1.6% of the above solution.

This solution is placed in mineral oil at a ratio of 1 partcatalase/prepolymer solution to 5 parts mineral oil and is rapidlyagitated with a motorized mixer to form an emulsion. This emulsion isilluminated with a long-wavelength ultraviolet light (360 nm) for 5 minto crosslink the PEG prepolymer to form a gel. The mw of the prepolymermay be selected to resist the diffusion of the catalase from the gel,with smaller PEG DA molecular weights giving less diffusion.

PEG DA of MW 10,000, further crosslinked with PEG DA 1000, shouldpossess the appropriate permselectivity to restrict catalase diffusion,and it should possess the appropriate permselectivity to permit thediffusion of hydrogen peroxide into the gel-entrapped catalase to allowthe enzymatic removal of the hydrogen peroxide from the bloodstream.Furthermore, it should possess the appropriate biocompatibility tocirculate within the bloodstream.

In this way, the gel is used for the controlled containment of abioactive agent within the body. The active agent (enzyme) is large andis retained within the gel, and the agent upon which it acts (substrate)is small and can diffuse into the enzyme rich compartment. However, theactive agent is prohibited from leaving the body or targeted bodycompartment because it cannot diffuse out of the gel compartment.

EXAMPLE 26 Cellular Microencapsulation for Evaluation of Anti-humanImmunodeficiency Virus Drugs In Vivo

HIV infected or uninfected human T-lymphoblastoid cells can beencapsulated into PEG gels as described for other cells above. Thesemicrocapsules can be implanted in a nonhuman animal to create a testsystem for anti-HIV drugs, and then treated with test drugs such as AZTor DDI. After treatment, the microcapsules can be harvested and theencapsulated cells screened for viability and functional normalcy usinga fluorescein diacetate/ethidium bromide live/dead cell assay. Survivalof infected cells indicates successful action of the drug. Lack ofbiocompatibility is a documented problem in this approach to drugevaluations, but can be overcome by using the gels described herein.

Modifications and variations are obvious from the foregoing detaileddescription. Such modifications and variations are intended to comewithin the scope of the following claims.

1. A substrate comprising a surface having a polymeric coating thereonformed by free radical polymerization of a biocompatible, substantiallywater soluble macromer comprising at least two free radicalpolymerizable substituents, wherein the coating further comprises one ormore polysaccharides wherein the substrate is an implantable material.2. The substrate of claim 1, wherein the implantable material isselected from the group consisting of woven material, a velour and anexpanded membrane.
 3. The substrate of claim 1, wherein the macromer ispoly(ethylene glycol) and the free radical polymerizable substituentscomprise carbon-carbon double bonds.
 4. The substrate of claim 1,wherein the polymeric coating is formed on the substrate surface by: a)applying to the surface the macromer and a free radical polymerizationinitiator; and b) exposing the initiator to an agent to activate theinitiator to cause the polymerization of the macromer to form thepolymeric coating on the surface.
 5. The substrate of claim 4, whereinthe initiator is selected from the group consisting of visible light orlong wavelength ultraviolet light-activatable free radical initiators,thermal activatable free radical initiators, benzoyl peroxide, potassiumpersulfate and ammonium persulfate.
 6. The substrate of claim 1, whereinthe polymeric coating is formed on the substrate surface by: a) applyingto the surface a free radical polymerization initiator with the macromerto form a mixture; and b) exposing the mixture to an agent to activatethe initiator to cause the polymerization of the macromer to form thepolymeric coating on the surface.
 7. The substrate of claim 6, whereinthe initiator is selected from the group consisting of visible light orlong wavelength ultraviolet light-activatable free radical initiators,thermal activatable free radical initiators, benzoyl peroxide, potassiumpersulfate and ammonium persulfate.
 8. The substrate of claim 1, whereinthe polysaccharide is selected from the group consisting of alginate,hyaluronic acid, chondroitin sulfate, dextran, dextran sulfate, heparin,heparin sulfate, heparan sulfate, chitosan, gellan gum, xanthan gum,guar gum, water soluble cellulose derivatives, and K-carrageenan.